EXCLUSIVE: Colin Firth has joined Reese Witherspoon in Devil’s Knot, the Atom Egoyan-directed drama about the West Memphis 3. As Deadline revealed just after the West Memphis 3 murder defendants were released from prison August 19, The Sweet Hereafter and Chloe director Egoyan came aboard to direct a script that was originally written by Scott Derrickson and Paul Boardman, the team behind The Exorcism Of Emily Rose.

Firth will play Ron Lax, a private investigator who was the first pro bono supporter of the defendents as they headed to trial in 1993. Lax built one of the most prominent private investigative firms in the Southeast, and offered his services for free to the defendants, who at the time were reviled because of the heinous nature of the crime and the sensationalized reports about devil worship and ritualistic sacrifice. All of that was later proven to be unfounded, after Damien Echols, Jason Baldwin and Jessie Misskelley were convicted despite not a shred of physical evidence. Lax was an unsung hero and helped find the DNA in the knots that bound one of the victims, that cast suspicion on Terry Hobbs, the stepfather of Steven Branch, one of the 8-year old murdered boys. Witherspoon will play Pam Hobbs, the mother of Branch. She initially believed the trio murdered her son, is eventually persuaded that the three suspects are innocent and wrongly accused. Filming begins next summer in Louisiana, and other big names are expected to join the cast in smaller roles.

Egoyan has been working hard with Boardman on a rewrite. The script is based on investigative reporter Mara Leveritt’s 2003 book Devil’s Knot: The True Story of the West Memphis Three, an in-depth chronicle of the sensationalized trials that sent the three to prison for the murder of the three 8-year old boys who were found hog-tied in a drainage ditch. The project is not a rush job. Derrickson and Boardman began writing it in 2006 when the film first took root at Dimension Films. It will be produced by Elizabeth Fowler, Clark Peterson, Richard Saperstein and Boardman. Saperstein was Dimension president and he acquired the film. After he left, Dimension eventually put the project into turnaround and Saperstein became re-involved as producer. The package includes life rights deals with some of the figures in the case, including Lax, who worked on the case for years. There are no rights deals with the West Memphis 3 defendants Damien Echols, Jason Baldwin and Jessie Misskelley Jr because at the time, two were serving life sentences and Echols was on death row. The producers are close to locking distribution and financing. It’s all coming together on the eve of Berlin, where the film should make some noise. Firth, who won the Best Actor Oscar for The King’s Speech, just starred with Cameron Diaz in the remake of Gambit, the Michael Hoffman-directed film scripted by Joel and Ethan Coen. CBS Films will release it. Firth’s repped by CAA.